Brushless DC motor (BLDCM) has a high performance, easy to control, no commutation sparks and excitation loss, long life and many other advantages, so the range of applications across various fields, have a good market prospects. Introduced a variety of major chip companies brushless DC motor control chip, such as STMmicore ectronics the ST72262 and ST7MC, developed by the U.S. Semiconductor MC33035, Japan Sanyo LB1690 and some other products, as well as common use of DSP chips with PWM After many years of technical accumulation, these control technology is very mature.

Electric Bikes with this design, the circuit is stable, relatively dedicated chip, the cost is relatively low, the signal flow clear and easy to understand, which will help control for brushless DC motor understanding of the principles.

Brushless DC motor controller hardware design

Motor-driven part of the design

Permanent magnet brushless DC motor system into a rotor, the stator windings with a phase change to replace the brush commutation, the Hall element sensor with rotor position. Hall built three motor components in real time the output voltage signal to determine the three rotor position information to characterize the digital control circuit according to the different voltage signal, the drive circuit to achieve selective conduction, so that inspire the rotor coil magnetic moment, in a cycle, each MOS MOSFET 120 °.

Converter in the power MOS used in this design works its switching state, MOS tube has three pole G (gate), S (source), D (drain), MOS tube is an important parameter is the VT , said the turn-on voltage, when the gate-source voltage VGS
Make the motor turn up, the Ministry must first control the induction to the Hall sensor rotor current location, then the stator winding in accordance with the decision on (or off) converter (inverter) power transistors in the order [1], Figure 1:
inverter in the T1, T3, T5 (these are called arm-power transistors, which can be N but also P-type tube-tube) and T2, T4, T6 (these are called the lower arm power transistors, only N-type tube), so sequence generated by the coil current flowing through the motor forward (or backward) rotating magnetic field, and interaction with the rotor magnet, so you can make the motor clockwise / counterclockwise rotation. When the motor rotor to the Hall sensor sensing the position of another group of signals, the control unit again on the next set of power transistors, and so the motor can continue to rotate in one direction until the control of the Ministry decided to stop the rotor power is turned off transistor (or just under the arm to open the power transistor); to reverse the rotor power transistor on the opposite order.

Decoding part of the design and control of motor

Decoding circuit circuit is divided into parts, and over-current protection circuit part of the governor, part decoding circuit (Figure 2) to protect the motor runs continuously, the control circuit part is mainly used for motor speed control and over-current protection status and battery-related the display.
"0" means low, "1" high, the motor when energized, the torque starts to use the initial position after the rotation process of Hall signals from the motor through the decoding circuit to generate a steady stream of drive signals corresponding to select the power MOS turn-on and turn-off (ie, off state), to achieve continuous operation of the motor [2], see table:
Taking into account the safety of the motor to run continuously to prevent cross-conduction phase change, if this happens, two on simultaneously, the two on the voltage drop is very small, as power is applied directly on the wire as an instant more than 45A of current, MOS tube is bound to be destroyed, so the same drive axle of the two MOS tube can not turn, can only turn one of them, in the design, the use of "interlock" program to prevent such from happening. "Interlocking" the principle is the same on the drive tube when there is a turn-on, the other a "G" (gate) voltage must be "0" (low level), in the event of non-normal and "G" voltage should be low, but actually it is high, it can be pulled low mandatory, practical applications can use a 74LS38 to achieve (74LS38 - Quad 2-input NAND buffer (OC)).

Another way to achieve, as shown in Figure 3 for the first P-type MOS tube IGBT, the second N-type IGBT.
The reason for using this design is to ensure that this is not the same time, the MOS turn-on and burnt, in the digital control part of the output is low, the first MOS tube conduction, and in case of no drive signal, while the TLP521 high input level, the conduction is not unexpected situations.

Part of the motor speed over-current protection circuit design

To achieve some of the major motor control motor speed control and overcurrent protection, and brake functions. Speed ??is of the Hall signal is sampled, the frequency of the signal changes according to the Hall to get that motor speed voltage, this voltage is input to the comparator, and the timing resistor voltage compared to determine the output is high or low, the motor acceleration and deceleration, schematic diagram shown in Figure 4.
To ensure the safe operation of the motor to make it through the current does not exceed the maximum tolerance value, so the design must be protected, over-current protection, under the principle is the drain current of MOS tube arm for sampling, a reasonable multiple of amplification After comparison with the reference voltage, if the sample voltage exceeds a safe value, the comparator output low voltage, the MOS tube end, so as to protect the motor function, as shown in Figure 5.
Two brake control function is to pick an output of the comparator switches to ground, so that more output is either high or low, as long as the switch is turned on, it sends down its potential direct, cut-off conduction of the MOS tube to achieve the purpose of the brake.
Battery status display circuit shown in Figure 6, the reference voltage is 12V, set limits battery discharge voltage of 30V, through the divider circuit and the reference voltage, if the output is low voltage, the LED is off, the battery needs charging, and otherwise the output is high, LED has been lit, you do not need charging.

Conclusion

Brushless motor has been very widely used in all areas of daily life, especially in recent years the booming clean, convenient electric folding bikes, the design mentioned in this article is not only suitable for electric bicycle brushless motor drive is also suitable for more high-power brushless motor drive. Control scheme using discrete devices, signal changes and signal flow is very clear, to facilitate future maintenance.